In general, tapered shank tool holders having a knob on the tapered end are releasably held within a tapered socket on a complementary, hollow rotating spindle. The spindle has an axial hole which penetrates through the tapered socket, collet fingers which grip the tapered shank tool holder knob, and a draw bar connected to the collet fingers, housed so as to be able to slide in the shaft hole. The collet fingers are turnable in the space between the knob-gripping clamped position and the knob-releasing unclamped position. The draw bar is movable in the axial direction of the spindle between an operating position in which the tool holder is held in the spindle tapered socket and a non-operating position, in which the tool holder is released from the tapered socket.
In order to reliably hold the tool holder in the spindle tapered socket, a bias is normally applied on the draw bar toward the operating position by a strong spring force. At this point, the collet finger rotation is arrested, and the collet fingers are stopped in the clamped position. By this means, the knob held by the collet fingers is pulled toward the draw bar operating position, and the tapered shank is tightly fit into the tapered socket.
When machining by a given tool is completed, the machine tool controller generates a tool release signal, which releases the tool from the spindle. In response to this tool release signal, a tool changing device grips the tool holder which has been inserted in the spindle using an arm. When the controller receives a confirmation signal indicating that the tool changing arm has gripped the tool holder, it commands the draw bar to move against the spring force to the non-operating position using appropriate motion means, such as an air cylinder. By moving the draw bar to the non-operating position, tensile force from the spring is removed form the knob. The arresting of the rotation of the collet fingers ends with the movement of the draw bar. Nevertheless, the tool holder remains in the grip of the tool changing device arm, so that it will not fall down due to its weight.
However, a machine tool controller of this type cannot command the draw bar to move until a confirmation signal has been received. This is because when the draw bar moves to the non-operating position, i.e., until the tool changing device arm has gripped the tool holder, the weight of the tool holder causes the knob thereof to be released from the collet fingers, and a tapered shank may separate from the spindle tapered socket inside surface. Thus, in order to reliably prevent the tool holder from falling, the machine tool controller must wait for a confirmation signal. This wait time reduces tool changing operation efficiency.